1. Field of the Invention
The present invention relates to a method of manufacturing a multi-domain vertical alignment (MVA) liquid crystal display device and to a multi-domain vertical alignment liquid crystal display device in which liquid crystals with negative dielectric anisotropy are sealed between two substrates, which are placed to face each other.
2. Description of the Prior Art
Liquid crystal display devices have the advantages that they are thin and light and that they can be operative at low voltages and have low power consumption. Accordingly, liquid crystal display devices are widely used in various kinds of electronic devices. In particular, active matrix liquid crystal display devices, in which a thin film transistor (TFT) is provided as a switching element for each picture element, show excellent display quality, which is comparable to that of cathode-ray tube (CRT) displays, and therefore have come to be widely used for displays for televisions, personal computers, and the like.
In general, a liquid crystal display device has a structure in which liquid crystals are sealed between two substrates, which are placed to face each other. A TFT, a picture element electrode, and the like are formed on one substrate, while color filters, a common electrode, and the like are formed on the other substrate. Hereinafter, the substrate on which a TFT, a picture element electrode, and the like are formed is referred to as a TFT substrate; and the substrate, which is disposed to face the TFT substrate, is referred to as an opposite substrate. A structure formed by sealing liquid crystals between the TFT substrate and the opposite substrate is referred to as a liquid crystal panel.
Heretofore, twisted nematic (TN) liquid crystal display devices have been widely used, in which liquid crystals with positive dielectric anisotropy are sealed between two substrates, and in which liquid crystal molecules are twisted and aligned. However, TN liquid crystal display devices have a disadvantage that viewing angle characteristics are poor and that contrast and color greatly change when the screen is viewed from an oblique direction. Accordingly, multi-domain vertical alignment (MVA) liquid crystal display devices, which have favorable viewing angle characteristics, have been developed and put to practical use.
FIGS. 1A and 1B are cross-sectional schematic views showing an example of an MVA liquid crystal display device. A TFT substrate 10 and a opposite substrate 20 are placed with spacers (not shown) interposed therebetween, and liquid crystals 30 with negative dielectric anisotropy are sealed between these two substrates 10 and 20. On a picture element electrode 12 of the TFT substrate 10, slits 12a are provided as alignment control structures to determine the oblique direction of liquid crystals when applying a voltage. The surface of the picture element electrode 12 is covered with a vertical alignment film 14 made of, for example, polyimide.
Meanwhile, a plurality of bank-like protrusions 23 are also formed as alignment control structures under a common electrode 22 of the opposite substrate 20. These protrusions 23 are placed at positions obliquely deviated from the slits 12a of the picture element electrode 12 as shown in FIG. 1A. The surfaces of the common electrode 22 and the protrusions 23 are also covered with a vertical alignment film 24 made of, for example, polyimide.
Polarizing plates (not shown) are placed under the TFT substrate 10 and over the opposite substrate 20, respectively, in a way that absorption axes are orthogonal to each other. A backlight (not shown) is also fastened under the TFT substrate 10.
In the MVA liquid crystal display device made up as described above, in the state where a voltage is not applied between the picture element electrode 12 and the common electrode 22, liquid crystal molecules 30a are aligned perpendicular to the substrate surfaces as shown in FIG. 1A. However, the liquid crystal molecules 30a in the vicinities of the protrusions 23 are aligned in directions perpendicular to the inclined surfaces of the protrusions 23. In this case, the light, which goes into a liquid crystal layer through polarizing plates from the downside of the TFT substrate 10, passes through the liquid crystal layer without any change in polarizing direction, and is blocked by the polarizing plate placed over the opposite substrate 20. In this case, a black display is produced.
When a voltage is applied between the picture element electrode 12 and the common electrode 22, the liquid crystal molecules 30 are inclined at an angle that corresponds to the applied voltage. In this case, as shown in FIG. 1B, the oblique directions of the liquid crystal molecules 30a are different on both sides of each of the slits 12a and the protrusions 23. Thus, what is called alignment division (multi-domain) is achieved. As shown in FIG. 1B, when the liquid crystal molecules 30a are aligned with an oblique direction relative to the substrate surfaces, light going into the liquid crystal layer through the polarizing plate from the downside of the TFT substrate 10 changes in polarizing direction in the liquid crystal layer, and passes through the polarizing plate which is placed over the opposite substrate 20. The amount of light passing through the polarizing plate depends on a voltage which is applied between the picture element electrode 12 and the common electrode 22.
As shown in this FIG. 1B, in the MVA liquid crystal display device, the tilt directions of the liquid crystal molecules 30a are different on both sides of each of the slits 12a and the protrusions 23 when a voltage is applied. Accordingly, the leakage of light in oblique directions is suppressed, and excellent viewing angle characteristics can be obtained.
In the MVA liquid crystal display device, since the tilt directions of the liquid crystal molecules are not determined by an alignment film, a rubbing process and a washing process for removing dust generated by the rubbing process, which are required for a TN liquid crystal display device, are eliminated; and therefore it has the advantage that the number of manufacturing steps are reduced. Additionally, non-uniformity in display caused by non-uniformity in the alignments of the liquid crystal molecules, which arises out of a rubbing process, is also suppressed.
As a method of manufacturing a liquid crystal display device which does not require a rubbing process, Patent Publication No. 3520376 describes that liquid crystals having been added with a polymer constituent (polymer molecule precursor) are sealed between a pair of substrates, and thereafter the polymer constituent is polymerized by irradiation of ultraviolet to form an alignment film. Japanese Patent Application Laid-open No. Hei 6-160818 describes a manufacturing a liquid crystal element that liquid crystals having been added with a polymer constituent are sealed between a pair of substrates, and thereafter the polymer constituent is polymerized to form a plurality of domains with various kinds of characteristics which are different from each other.
Incidentally, in a conventional MVA liquid crystal display device, the phenomenon occurs in which the screen looks whitish when viewed from an oblique direction. FIG. 2 is a view showing T-V (transmittance-voltage) characteristics for the case where the screen is viewed from the front and those for the case where the screen is viewed from the oblique direction (azimuth 45° and polar angle 60°), with applied voltage (V) on the horizontal axis and transmittance on the vertical axis. In the present application, an angle formed by a line, which is a projection of a line of sight on a liquid crystal panel, and the X-axis of the liquid crystal panel (a line extended from the center, as the origin, of the liquid crystal panel to the right direction) is referred to as azimuth, while an angle formed by a line normal to the liquid crystal panel and a line of sight is referred to as polar angle.
As shown in FIG. 2, in the case where a voltage slightly higher than a threshold voltage of the T-V characteristics (hereinafter, simply referred to as “the threshold voltage”) is applied to the picture element electrode, the transmittance when the screen is viewed from the oblique direction is higher than that when the screen is viewed from the front. Further, when the applied voltage becomes high to some extent, the transmittance when the screen is viewed from the oblique direction becomes lower than that when the screen is viewed from the front. Accordingly, differences in brightness between red, green, and blue picture elements become small when the screen is viewed from the oblique direction. As a result, the phenomenon in which the screen looks whitish occurs as described previously. This phenomenon occurs not only in MVA liquid crystal display devices but also in TN liquid crystal display devices.
In the specification of U.S. Pat. No. 4,840,460, a technology is proposed in which a plurality of sub picture element electrodes are provided in one picture element and in which these sub picture elements are capacitively coupled. In such a liquid crystal display device, since a voltage to be applied on each sub picture element electrode is determined in accordance with the capacitance between the sub picture element electrodes, different voltages can be applied to the sub picture elements, respectively. Accordingly, it appears that a plurality of regions having different threshold voltages from one another exist in one picture element region. In the case where a plurality of regions having different threshold voltages from one another exist in one picture element region as described above, the T-V characteristics of the picture element region is represented as a composition of the T-V characteristics of the respective sub picture element regions. AS a result, the phenomenon is suppressed in which the transmittance when viewed from the oblique direction becomes higher than that when viewed form the front, and the phenomenon in which the screen looks whitish is also suppressed.
In Japanese Patent Application Laid-open No. Hei 5-66412, a liquid crystal display device is disclosed in which, in order to improve the viewing angle characteristics, each picture element electrode is divided into a plurality of sub picture element electrodes and in which control electrodes are respectively placed under the sub picture element electrodes with an insulating film interposed therebetween. In this liquid crystal display device, the control electrode and the sub picture element electrode are capacitively coupled with the insulating film. The same voltage is applied to each of the control electrodes through a TFT, and a voltage in accordance with a capacitance between the control electrode and the sub picture element electrode is applied to each sub picture element electrode.
As described in these above public documents, a method in which display characteristics are improved by dividing one picture element region into a plurality of sub picture element regions having different T-V characteristics is called a halftone grayscale (HT) method.
However, the inventors of the present patent application view the above-described HT method by capacitive coupling as having the following problems. That is, the HT method by capacitive coupling has a drawback that it is necessary to redesign an exposure mask and the like to be used at a time of manufacturing. In addition, the HT method by capacitive coupling needs to have a control electrode to be capacitive-coupled to a sub picture element electrode. In general, since a control electrode is formed at the same time as the forming of a source and a drain electrodes or a gate electrode of a TFT, it is made of metal such as aluminum (Al) and has a light blocking effect. Accordingly, the control electrode causes a decrease in the aperture ratio of a picture element, and the screen becomes dark. Although it is conceivable that a control electrode is formed of a transparent conductive material such as indium-tin oxide (ITO), the number of manufacturing steps will increase in that case.